Apparatus for input amplifier stage

ABSTRACT

A differential stage of an amplifier includes an input transistor and a second transistor with a significantly greater emitter cross-sectional area than the input transistor. Quiescent emitter current is divided between the two transistors in substantially the same ratio as the emitter cross-sectional areas so that the emitter current densities are substantially equal.

This application is a continuation of application Ser. No. 348,661,filed Feb. 12, 1982, now abandoned, which was a division of applicationSer. No. 194,124, filed Oct. 6, 1980, which issued as U.S. Pat. No.4,371,847 which was a division of application Ser. No. 020,204 filedMar. 13, 1979, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to data transmission links andin one of its aspects to a fiber optic data transmission link to bepowered by low voltage single polarity power supplies with unreliablevoltage levels over a wide range of temperatures.

2. Description of the Prior Art

Control systems such as control systems for airplanes are often of aconventional wire connected electrical nature. Wire connected electricalcontrol systems are subject to electrical noise picked up by the controlcables. Other conducted electrical data transmission links are subjectto similar problems. One way of obviating the problem of electricalnoise picked up by the transmission cables is to transmit data opticallyrather than electrically. One way of transmitting optical data is by theuse of fiber optics cables.

One way of generating optical data is by the use of a light emittingdevice such as a light emitting diode known as an LED. The LED is a lowimpedance device when operated under forward bias and exhibits anegative temperature coefficient. Thus, to avoid avoid thermal runaway,the LED can be driven from a high impedance source, ideally a constantcurrent source. Control systems are often used in environments where thepower supply is low voltage and produces output with an unreliablevoltage level, and temperatures vary over a wide range, so the LEDtransmitter must regulate the LED current against changes in supplyvoltage and changes in temperature.

Most amplifier work in the past has been done in "matching systems",systems where the transmitting medium such as a cable or an antenna islargely resistive in nature so that the transmitters and receivers aredesigned to match the impedance of the cable or antenna. Photodiodes, onthe other hand, are highly capacitive in nature.

In order to put an entire transmitter or an entire receiver upon asingle chip, it is necessary to make the transmitter or receiver and thelogic of compatible integrated circuit technology. Typically the logiccircuitry is transistor-transistor logic known as TTL. TTL is almostalways used to transmit unipolar signals, but data is often most easilytransmitted by a bipolar system, especially for transmitting codes withapproximately a 50 percent duty cycle such as a Manchester code sincethe problems of dc voltage offsets can be avoided.

A common restriction, especially for systems designed for the military,is that the system be able to operate on a positive power supply only.Additionally, most of the bipolar technology industry is geared forproducing npn technology only.

There is very little problem with noise picked up in transmission infiber optics systems, so that the noise generated from the system itselfdetermines the performance of the system.

SUMMARY OF THE INVENTION

The present invention concerns a method and apparatus for transmittingconducted electrical data by converting the conducted electrical datainto electromagnetic radiated wave data such as microwave or opticaldata and transmitting the optical data, for example, to a receiver whichreconverts the optical data into conducted electrical data. A fiberoptic data transmission link according to the present invention isdesigned to be used in conjunction with at least one power supply andincludes a transmitter powered by the at least one power supply foracquiring conducted electrical data and transmitting optical datacorresponding to the conducted electrical data, a receiver powered bythe at least one power supply for receiving optical data and generatingconducted electrical data corresponding to the optical data, and atleast one fiber optic data transmission cable for transmitting theoptical data from the transmitter to the receiver. Often a fiber opticdata transmission link according to the present invention is used toelectrically isolate one piece of equipment from another. In such a caseit is preferable that the at least one power supply comprise at leasttwo power supplies with no power supply in common between thetransmitter and the receiver.

Power and reference voltages are supplied by a temperature compensatedvoltage reference circuit and are substantially constant in thetemperature range from -55° C. to +150° C. and in the range of powersupply voltage from ten percent below the nominal power supply voltageto ten percent above the expected power supply voltage. The relativecurrent in two branches of the voltage reference circuit is determinedby the relative cross-sectional area of the emitters of transistors inthe two branches.

A transmitter according to the present invention includes an input gatefor accepting conducted electrical data from a source outside of thetransmitter, a light emitting device for emitting a significant amountof light to the at least one fiber optic data transmission cable whensupplied with sufficient electric current, and an output driver forsupplying the light emitting device with sufficient electric current foremitting a significant amount of light responsive to the conductedelectrical data accepted by the input gate.

A transmitter according to the present invention also includes atemperature compensated voltage reference circuit for supplying areference voltage. The output driver includes a plurality of resistorsof predetermined differing values and a means for applying the referencevoltage across a predetermined resistance made up of at least onepreselected resistor of the plurality of resistors whereby the currentsupplied to the light emitting device by the output driver when theoutput driver is on is substantially a predetermined constant. Bothlinear circuitry and logic circuitry can be included on the same chip.

One arrangement of a transmitter according to the present invention alsoincludes a pre-bias current source for continuously supplying electriccurrent to the light emitting device. The amount of current supplied bythe pre-bias current source is less than the amount of currentsufficient for the light emitting device to emit a significant amount oflight, but the substantially constant predetermined current supplied bythe output driver when the output driver is on added to the current fromthe pre-bias current source is sufficient for the light emitting deviceto emit a significant amount of light so that the light emitting deviceis responsive to the output driver. By keeping the light emitting deviceturned on at a low level at all times it can respond more quickly to theoutput driver.

Another arrangement of a transmitter according to the present inventionincludes an encoder, such as a Manchester encoder, between the inputgate and the output driver in order for the receiver to accept anon-return-to-zero code. By use of the encoder the non-return-to-zerocode is converted to a return-to-zero code with a substantially fiftypercent duty cycle for transmission.

The receiver comprises the combination of a means for detecting theoptical data and generating a corresponding electrical signal, apreamplifier for acquiring the electrical signal as an input andgenerating bipolar electrical data corresponding to the electricalsignal input, a postamplifier for amplifying the conducted electricaldata generated by the preamplifier, and an output stage which includes acurrent mode comparator for acquiring the amplified bipolar electricaldata from the postamplifier as an input and generating single-endedconducted electrical data corresponding to the bipolar electrical dataas an output. The preamplifier utilizes automatic gain control in orderto take into account wide variations in the amplitude of the receiversignal while preventing the stretching of signals due to over drivingany amplifier stages. A preferred embodiment of the receiver alsoincludes a reference preamplifier that substantially duplicates thepreamplifier and ensures that the preamplifier and the automatic gaincontrol circuit are supplied with the proper currents.

The preamplifier includes an input amplifier for amplifying the inputconducted electrical data from the photodiode. The automatic gaincontrol circuit supplies a negative feedback from the output of theinput amplifier to the input of the input amplifier and operates so thatthe effective feedback resistance decreases as the output amplitudeincreases and increases as the output amplitude decreases so that theelectrical gain of the preamplifier decreases as the output amplitudeincreases and increases as the output amplitude decreases. The automaticgain control circuit includes an automatic gain control capacitor, thevoltage across the capacitor being responsive to the output of the inputamplifier, and a current divider with two branches. The first branch ofthe current divider is connected to the inverting input of the inputamplifier and the second branch of the current divider is connected to aconvenient node in the circuit that has a voltage sufficient to keep thesecond branch in an active condition. The automatic gain control circuituses the difference between the voltage across the automatic gaincontrol capacitor and a reference voltage to control the ratio of thecurrent split between the two branches. The difference between the twovoltages is adjusted so that the quiescent current in the second branchis significantly greater than the quiescent current in the first branchby a predetermined amount.

The postamplifier includes at least one balanced differential amplifierwhich includes a means for limiting the difference in voltage betweenthe two differential outputs to a predetermined amount which is smallcompared to the voltage of the power supply thereby preventing theactive devices such as transistors from going too far into saturationand cut off which would distort the conducted electrical data. In apreferred embodiment, the differential amplifer is a transistordifferential amplifier and the means for limiting the difference involtage between the two differential outputs comprises a first diodeconnected in a forward biased direction from the output of onetransistor to the output of the other transistor and a second diodeconnected in the opposite direction. In one embodiment the postamplifierincludes two such differential amplifier stages in tandem with oneoutput of the first differential transistor stage resistively coupled toboth inputs of the second differential transistor stage, and the otheroutput of the first differential transistor stage capacitively coupledto only one input of the second differential transistor stage. Such anarrangement uses the inherent common mode rejection of the differentialstage to ac couple the two differential transistor stages with only onecapacitor.

One embodiment of the receiver also includes a status detector to detectthe presence of data signal. The output signals of the receiver and thestatus indicator can then be logically combined so that a final outputis obtained only when signal is present and is of an amplitude greaterthan the corresponding noise in the circuit.

A method of altering the high frequency electrical characteristics of anelectrical component in a monolithic or hybrid circuit, for example aresistor, includes surrounding the resistor on three sides by a guardingresistor so that there is almost no capacitance between the firstresistor and the substrate. The voltage across the guard resistor isdriven by the voltage across the first resistor by means of a followercircuit. In a more generalized case, the voltage across the guardresistor would be driven by two follower circuits, one at each end ofthe first resistor.

A method for making a low noise amplifier according to the presentinvention includes connecting one input of a differential amplifier to areference voltage, using the second input as the primary input to theamplifier, and making the emitter area of the transistor connected tothe reference input many times greater than the emitter area of theinput transistor. Thus, when the emitter current of the referencetransistor is many times greater than the emitter current of the inputtransistor, the emitter resistance of the reference transistor is afraction of the emitter resistance of the input transistor, and the gainand noise of the input stage are controlled by the input transistor, butthe dc offset and drift are eliminated as in a conventional differentialamplifier.

These and other objects, advantages and features of this invention willbe apparent from the following description taken with reference to theaccompanying drawings, wherein is shown the preferred embodiments of theinvention:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a fiber optic data transmission link inaccordance with the present invention;

FIG. 2 is a block diagram of an optic transmitter in accordance with thepresent invention;

FIG. 3 is a schematic diagram of the optic transmitter of FIG. 2;

FIG. 4 is a block diagram of an optic receiver in accordance with thepresent invention;

FIG. 5 is a schematic diagram of a receiver preamplifier according tothe present invention;

FIG. 6 is a schematic diagram of a receiver postamplifier and outputstage according to the present invention;

FIG. 7 is a cross-sectional elevation view of an integrated circuitresistor according to the prior art;

FIG. 8 is a cross-sectional elevation view of an integrated circuitresistor according to the present invention;

FIG. 9 is a top view of the resistor of FIG. 8; and

FIG. 10 is a schematic diagram of a status detector according to thepresent invention.

DETAILED DESCRIPTION

Referring now to FIG. 1, a fiber optic data transmission link inaccordance with the present invention is referred to generally byreference numeral 10. Data transmission link 10 is to be used inconjunction with at least one power supply 12 and includes a transmitter14 powered by at least one power supply 12 for receiving conductedelectrical data and transmitting optical data corresponding to theconducted electrical data, a receiver 16 powered by at least one powersupply 12 for receiving optical data and transmitting conductedelectrical data corresponding to the optical data, and at least onefiber optic data transmission cable 18 for transmitting the optical datafrom transmitter 14 to receiver 16. Data transmission link 10 alsoincludes means 20 for electrically connecting transmitter 14 to at leastone power supply 12, and means 22 for electrically connecting receiver16 to at least one power supply 12. Frequently a fiber optic datatransmission link is used to electrically isolate two pieces ofequipment from each other. In such a case it is preferable that at leastone power supply 12 comprise at least two power supplies with no powersupply in common between transmitter 14 and receiver 16 in order toensure electrical isolation. Transmitter 14 and receiver 16 are suppliedwith appropriate ground connections 15 and 17 respectively which may ormay not be common between the two pieces of equipment.

Referring now to FIGS. 2 and 3, transmitter 14 comprises in combinationa temperature compensated voltage reference circuit 24 for supplying areference voltage, a light emitting device such as LED 26 for emittinglight when supplied with a sufficient amount of electric current, and anoutput driver 28 responsive to the conducted electrical data accepted asan input by transmitter 14 for supplying electric current to lightemitter 26. Transmitter 14 also includes an isolation amplifier 30electrically connected between temperature compensated voltage referencecircuit 24 and output driver 28. Output driver 28 and amplifier 30together form a direct coupled feedback circuit that applies the outputvoltage of voltage reference circuit 24 to plurality of resistors 32.

Transmitter 14 further comprises an input gate 36 for driving outputdriver 28. Input gate 36 is responsive to the electrical data acceptedby transmitter 14. Input gate 36 includes an input terminal 38 foraccepting the electrical data, an enable terminal 40 for accepting anenable signal, and an output terminal 42. The electrical signal atoutput terminal 42 corresponds to the electrical data accepted at inputterminal 38 only when enable terminal 40 is enabled. Transmitter 14 canalso include an encoder such as a Manchester encoder 37 in order toaccept a non-return-to-zero (NRZ) code. Manchester encoder 37 isresponsive to NRZ electrical data accepted by transmitter 14 at inputterminal 39 when terminal 41 is enabled. Inverter 37a is included on theoutput of Manchester encoder 37 to create an output compatible with theoutput of input gate 36 so that the two outputs can be combined, in thiscase by a "wired and" 43a when Manchester encoder 37 is to be used.

Referring to FIG. 3, output driver 28 includes a plurality of resistors32 of predetermined differing values, and a means including transistorQ24 for applying the reference voltage across a predetermined resistancemade up of at least one of plurality of resistors 32. LED 26 isresponsive to output driver 28, and the current supplied to LED 26 byoutput driver 28 when the output driver is on, substantially equals apredetermined portion of the voltage at the output of voltage referencecircuit 24 divided by the predetermined resistance so that the currentsupplied to LED 26 by output driver 28 when the output driver is on, issubstantially a predetermined constant. When used in reference toplurality of resistors 32, "differing values" includes the situationwhere the resistors are of substantially the same value but can beconnected in varying configurations which have differing resultantresistance values. As illustrated in FIG. 3, the dashed lines extendingfrom the ends of plurality of resistors 32 represent connections thatcan be selectively made off the chip, thus putting selected resistorcombinations in parallel, having selected resultant resistance values.Off-chip resistor 32b can also be added to plurality of resistors 32.

One embodiment of transmitter 14 also comprises a pre-bias currentsource 34 for continuously supplying electric current to LED 26 in anamount less than the amount of current sufficient for LED 26 to emit asignificant amount of light. The substantially constant predeterminedcurrent supplied by output driver 28 when the output driver is on addedto the current from pre-bias current source 34 is sufficient for LED 26to emit a significant amount of light. Thus, LED 26 is responsive tooutput driver 28. The use of pre-bias current source 34 shortens theturn-on delay time for LED 26. This technique is generally usefuldespite the fact that LED 26 emits some small amount of light underpre-bias conditions. Although an LED does emit some light at even lowcurrent levels, it emits very little light at such low currents.

Temperature compensated voltage reference circuit 24 makes use of a verystable and predictable property of a silicon transistor, thebase-emitter voltage (V_(BE)), using V_(BE) as a reference to generate avoltage with a given temperature coefficient (TC) which becomes theoutput of the regulator. The term "temperature compensated" is used hereto describe an electrical output and a circuit for obtaining thatelectrical output which vary in a preselected manner with temperature.Often the preselected manner for the electrical output to vary withtemperature is for electrical output to make no substantial variationwith temperature over the rated temperature range. In the present case,the electrical output is the reference voltage. Voltage referencecircuit 24 includes transistor Q10, diode-connected transistor Q12connected between the base of Q10 and ground, and substantially equalresistors R12 and R14 connected between the regulated voltage and thecollectors of Q10 and Q12 respectively. Voltage reference circuit 24also includes resistor R10 connected between the emitter of Q10 andground. Transistors Q10 and Q12 are run at different emitter currentdensities (J). Voltage reference circuit 24 is of the general type knownas a band-gap regulator, but since the high voltage ends ofsubstantially equal resistors R12 and R14 are connected together and thelow voltage end are each one base to emitter voltage drop above ground,the current through R12 is the same as through R14 and, hence, theemitter current in Q10 is the same as in Q12, despite the fact that thecross-sectional area of the emitter of Q10 is ten times as great as thatof Q12, so the emitter current density of Q12 is ten times that of Q10.This difference in emitter current densities results in a difference inV_(BE) of: ##EQU1## where: T=°Kelvin

k=Boltzman's Constant

q=electronic charge

n=process constant

The difference in V_(BE) appears across R10 with a positive TC.Neglecting the transistor Q10 base current, the current in R12 equalsthe current in R10, so that the voltage across R12 is: ##EQU2## Theregulated voltage is the voltage at the emitter of Q14, which is:##EQU3## The TC of the regulated voltage is the sum of the positive TCacross R12 and the negative TC across V_(BE) of Q16. By adjusting theratio of R12 and R10, the regulated voltage can have zero TC or anyother selected value. The regulated voltage V_(EQ14) is thus temperaturecompensated.

In the past, band-gap regulators have used equal sized transistors Q10and Q12, and made resistor R12 many times large than resistor R14, tocorrespond to the present example the resistance of R12 would be tentimes the resistance of R14. Thus, the emitter current of Q10 is 1/10 ofthe current in Q12. Such an arrangement has several disadvantages: (1)the gain around the shunt feedback loop formed by transistors Q14 andQ16 with Q10 and Q12 is low so that the circuit is more responsive topower supply variations, (2) the low bias current in Q10 makes the noisegenerated in the regulator higher, and (3) the higher values ofresistors R12 and R16 use more area on the chip and, therefore, havemore capacitance and reduce the frequency response of the regulator. Thepresent invention, on the other hand, keeps the current in the threebranches the same by adjusting the emitter area of Q10 to lower thecurrent density. This eliminates the disadvantages listed for the priorart and results in an improved version of the band-gap regulator.

In order to get voltage compensation for variations in supply voltage,temperature compensated voltage reference circuit 24 also includestransistors Q14 and Q18 as well as resistors R16 and R18. The term"voltage compensated" is used here to describe an electrical output anda circuit for obtaining that electrical output which vary in apreselected manner with supply voltage. As with temperaturecompensation, the preselected manner is often no substantial variationover the rated power supply voltage range. Transistor Q14 was added topartially decouple supply variations from the regulated voltageV_(EQ14). The voltage at the emitter of transistor Q14 is, however,still somewhat responsive to variations in supply voltage since the gainaround the feedback loop made up of R12, Q16, R18 and Q14 is notinfinite, thus allowing some error voltage at the base of transistorQ14. The value of resistor R16 is selected to make the collector currentof transistor Q16 substantially the same as the collector current of Q10and Q12. The value of resistor R18 is then selected to compensate thevoltage at the collector of transistor Q16 against variations in supplyvoltage by making the ratio of the resistance of R18 to the resistanceof R16 such that it gives the smallest variation in the voltage at thecollector of transistor Q16, over the rated temperature and voltagerange. Reasonable resistor values can be chosen through iterativecircuit analysis. An output emitter follower made up of Q18 provides adriving point that is also temperature compensated since the TC at theemitter of Q18 equals the TC at the emitter of Q14. A resistor R20 canbe added at the base of Q18 to squelch oscillations resulting fromdriving a largely capacitive load with Q18 in an emitter followerconfiguration as is well known in the art. Thus, the voltage at theemitter of Q18 is compensated for both temperature and supply voltagevariations. Typical resistance values for resistors R10, R12, R14, R16and R18 consistent with the present example are 10 ohms, 3433 ohms, 3433ohms, 6585 ohms, and 253 ohms respectively.

Pre-bias current source 34 comprises a diode-connected transistor Q20and a resistor R22 connected in series between the reference voltageV_(EQ18) and ground. Pre-bias current source 34 further includes atransistor Q22 for supplying the current sink pre-bias current to LED26, and a resistor R24 connected to the emitter of transistor Q22.Without resistor R24, transistor Q22 would try to supply an emittercurrent equal to that in transistor Q20. The inclusion of R24 ensuresthat the difference in V_(BE) between the two transistors is droppedacross R24, thus determining the pre-bias current. Typical resistancevalues for R22 and R24 consistent with the present example are 1.56Kohms and 100K ohms respectively.

Isolation amplifier 30 separates the large current variations at outputterminal 42 and the base of output transistor Q24 from the output oftemperature compensated voltage reference circuit 24. Isolationamplifier 30 includes two transistors, Q26 and Q28, and three resistors,R84, R86 and R88, connected in a differential amplifier configurationwith the base of Q26 connected to the reference voltage at the emitterof Q18. Typical resistance values consistent with the present exampleare: R84=100 ohms, R86=390 ohms, and R88 =87 ohms. Output terminal 42 ofinput gate 36 is connected to the base of transistor Q24. The collectorof Q28 is connected to the base of output transistor Q24, and theemitter of output transistor Q24 is connected to the base of Q28,forming a tight feedback loop which maintains the emitter of outputtransistor Q24 at the reference voltage when output terminal 42 is high.Output driver 28 is thus responsive to the electrical data received bytransmitter 14 at input terminal 38 when enable terminal 40 is enabled.

When the signal at output terminal 42 is low, it pulls down on the baseof Q24, opening the feedback loop thus making the voltage at the emitterof Q24 low so that very little current flows through the collector ofQ24 and, hence, through LED 26. The connection of output terminal 42 tothe base of transistor Q24 allows LED 26 to be turned on and off with asmaller control current from output terminal 42 than would be possibleif output terminal 42 were connected directly in the LED drive currentpath. A minimum power dissipation in the drive circuit is thus ensured.

Referring now to FIGS. 4, 5 and 6, receiver 16 includes a means such asphotodiode 50 for receiving the optical data and generating conductedelectrical data corresponding to the optical data, a preamplifier 44 foracquiring the conducted electrical data generated by the photodiode 50and generating automatic gain controlled, amplified conducted electricaldata corresponding to the conducted electrical data accepted fromphotodiode 50, a postamplifier 46 for amplifying and shaping theconducted electrical data generated by preamplifier 44, and an outputstage 48 for acquiring the amplified conducted electrical data frompostamplifier 46 and transmitting the conducted electrical data. In apreferred embodiment, preamplifier 44 generates bipolar electrical datacorresponding to the data accepted by the preamplifier, and output stage48 transmits single-ended conducted electrical data corresponding to thebipolar electrical data accepted as an input from postamplifier 46.Optical data is, thus, received by photodiode 50, and then convertedinto conducted electrical data. The conducted electrical data is thenamplified by preamplifier 44 which adjusts its gain according to theamplitude of the signal. The conducted electrical signal is then furtheramplified and shaped by postamplifier 46, and, finally, the signal isconverted to a form suitable for output, such as TTL compatible, byoutput stage 48.

In one embodiment, preamplifier 44 includes an input amplifier 52 foramplifying the input conducted electrical data. The electrical gain ofpreamplifier 44 is responsive to the amplitude of the input conductedelectrical data, the gain decreasing as the amplitude increases andincreasing as the amplitude decreases. One way of accomplishing thischange in gain is by means of an automatic gain control circuit 54supplying a negative feedback from output 56 of input amplifier 52 toinput 58 of input amplifier 52. The effective feedback resistancedecreases as the output amplitude increases and increases as the outputamplitude decreases so that the electrical gain of preamplifier 44decreases as the output amplitude increases and increases as the outputamplitude decreases.

Automatic gain control circuit 54 includes a follower circuit 60 forfollowing output 56 of input amplifier 52, and an automatic gain controlcapacitor 62 which would normally not be included on the chip, thevoltage across capacitor 62 being responsive to output 56. Automaticgain control circuit 54 also includes a resistor R26 connected betweenthe output of follower 60 and capacitor 62 so that the voltage responseacross capacitor 62 will be the typical voltage response across acapacitor in a series RC circuit. Automatic gain control circuit 54 alsoincludes a current divider including a first current branch made up oftransistor Q30 and a second current branch made up of transistor Q32.Automatic gain control circuit 54 further includes a constant currentsource 64, a resistor R28 and a diode D10 connected in series betweenoutput 56 of input amplifier 52 and current source 64, as well as aresistor R30 connected from the emitters of Q30 and Q32 to constantcurrent source 64. One suitable constant current source such as forconstant current source 64 and others used in this circuit is providedby a current mirror. D10 is preferably a diode-connected transistor withan emitter area much greater than that of Q32, and R28 should have amuch lower resistance value than R30. Typical values consistent with theexample given would be the emitter of D10 being 20 times as large asthat for Q32 and R30 being 1,000 ohms, and R28 being 50 ohms or 1/20 ofR30. If constant current source 64 draws one milliamp, then roughtly0.05 milliamp will be drawn through R30 since the voltage drop V_(BE) ofQ32 will equal the voltage drop across D10 since Q32 and D10 arefabricated by the same process.

Referring to the loop comprising follower 60, resistor R26, base toemitter of transistor Q32, R30 and its parallel capacitor 84, diode D10and resistor R28, the dc bias conditions are selected so that a rise involtage across follower 60 offsets the voltage drop across resistor R26and there is no net voltage drop from the input of follower 60 to thebase of transistor Q32 so that transistor Q32 actually receives its dcbase current from the circuitry of follower circuit 68. Therefore, thevoltage drop from the base of transistor Q32 to the output of constantcurrent source 64 is the same as the voltage drop across resistor R28and diode D10. Since resistor R28 is 1/20 the size of resistor R30 anddiode D10 is a diode connected transistor with an emitter cross-sectionarea 20 times greater than that of transistor Q32, and since diode D10and transistor Q32 are made on the same chip by the same process andresistors R28 and R30 are made on the same chip by the same process, thevoltage drop across diode D10 tracks the voltage drop across the base toemitter of transistor Q32 with changes in temperature. Thus, the currentthrough resistor R30 remains 1/20 of the current through resistor R28.As will be more fully explained later, the quiescent collector currentof transistor Q30 is a predetermined current, in the present example 2.5microamps. The base of transistor Q30 is connected directly to the 1.7volt reference voltage and the base of transistor Q32 is connected toautomatic gain control capacitor 62. It is the difference between thebase voltages of the two current branch transistors that determines thecurrent split between them, the total emitter current of the twotransistors equaling 1/20 of the current through resistor R28 and diodeD10. Follower circuit 60 is constructed to supply temperature dependentcurrent to transistor Q32 and follower circuit 68 so that the automaticgain control voltage across capacitor 62 is substantially independent oftemperature. The automatic gain control circuit, therefore, respondsonly to the average value of input current from photodiode 50. It canthus be seen that as the voltage across automatic gain control capacitor62 changes, it will shift the balance between the currents drawn throughQ30 and Q32 so that more current is drawn through Q30, making theeffective feedback resistance look smaller, thus decreasing the overallgain of preamplifier 44.

The automatic gain control circuit operates on the preamplifier itselfto extend the optical signal range of the receiver beyond the normalrange obtainable with the power supply voltage limitations discussed.The action of AGC capacitor 62 in the feedback loop is similar to the dcfeedback action. As light sensitive device 50 current increases, thevoltage across the AGC capacitor decreases by the average value of thevoltage at output 56, which is 1/2 the peak value of the output 56voltage. Because the voltage at the base of transistor Q32 drops towardground, transistor Q30 forward bias will increase because its basevoltage is constant, and a larger percentage of the current in resistorR30 will not flow in transistor Q30. Thus, the effective feedbackresistance seen by the input has decreased. The trans-resistance ofpreamplifier 44 is reduced. Varying the trans-resistance is equivalentto changing the "gain" of preamplifier 44, and thus the nomenclature"automatic gain control loop". Since the voltage across AGC capacitor 62is equal to 1/2 the peak output voltage swing, the trans-resistance willgo from 1/20 to one for peak output swing of 0.0 to 0.5 volts. At 0.5volts R30 appears to be 1,000 ohms.

Input amplifier 52 comprises a differential input stage including aninput transistor Q34, which is a Schottky transistor in a preferredembodiment, and a bipolar transistor Q36. The base of transistor Q36 iscontrolled by the 1.7 volt reference voltage, and the emittercross-sectional area of Q36 is significantly greater than the emittercross-sectional area of Q34 so that the quiescent emitter current oftransistor Q34 is significantly less than the quiescent emitter currentof bipolar transistor Q36 when the transistors are biased to the samebase to emitter voltages. In this case, transistor Q36 has ten times thecross-sectional emitter area of transistor Q34 so that transistor Q36has ten times the quiescent emitter current of transistor Q34. Thetransistor base to emitter voltages are balanced since the transistorsare created by the same process, thus providing drift cancellation. Theemitter resistance of Q36 is, however, 1/10 the emitter resistance ofQ34:

    r.sub.e =kT/qI.sub.e

where:

r_(e) =emitter resistance

T=°Kelvin

k=Boltzman's Constant

g=electronic charge

I_(e) =emitter current

Thus, the combined emitter resistance of the differential stagecomprised of Q34 and Q36 is only 10 percent greater than that of Q34alone. The trans-conductance of the differential stage is given by:

    g.sub.m =1/(r.sub.e 34 +r.sub.e 36)

and the mean square input noise voltage divided by the noise bandwidthis

    (e.sub.n.sup.2 f.sub.BW)=2kT(r.sub.e 34+r.sub.e 36)

Since r_(e) 36 is 1/10 of r_(e) 34, it is clear that the g_(m) of thestage is decreased by only 10 percent compared to Q34 alone and the meansquare input noise voltage is increased by only 10 percent compared toQ34 alone. For a conventional differential stage where the twotransistors have the same emitter area, the g_(m) is reduced by a factorof two and the mean square input noise voltage is increased by a factorof two. The gain and noise of input amplifier 52 is thus controlled byQ34.

A preferred arrangement of preamplifier 44 includes a reference inputamplifier 86 designed to supply the correct base current to transistorQ34 of input amplifier 52 and the correct quiescent collector current totransistor Q30 of automatic gain control circuit 54. Again, all valuesused are by way of illustration and not of limitation. Reference inputamplifier 86 includes transistors Q34b and Q36b which are similar to Q34and Q36 respectively along with bias circuitry similar to that of inputamplifier 52. A 1K ohm resistor R33b is connected in series with a 40Kohm resistor 44b to the base of Q34b, and similarly a 1K ohm resistorR43 is connected in series with a 40K ohm resistor R44 to the base ofQ34. Reference input amplifier 86 includes a 100 microamp constantcurrent source 88 connected between R43b and R44b so as to create anadditional 1/10 volt drop across the 1K ohm resistor R43b. Sincetransistor Q34b is fabricated the same as transistor Q34 and biased thesame, it draws essentially the same base current as Q34. Since thevoltage at the common node between R43 and R43b supplies current to bothcircuits, the extra 1/10 of a volt dropped across R43b is dropped acrossthe combination of R43 and R44, providing the additional one voltdivided by 41K ohms or approximately 2.5 microamps. Thus, the currentthrough R43 and R44 supplies the temperature compensated base currentfor Q34 and an additional 2.5 microamps for the collector of transistorQ30 of automatic gain control circuit 54. Reference input amplifier 86also includes a transistor Q59b connected in a feedback relationshipbetween the collector of Q34b of reference input amplifier 86 and thecommon bias node between resistors R43 and R43b, thus regulating the dcbias voltage. The dc conditions of input amplifier 52 and referenceinput amplifier 86 are duplicated in such a manner that they track oneanother over temperature. A preferred form of reference input amplifier86 also includes a capacitor 90 connected between the base of inputtransistor Q34b and ground and a capacitor 92 connected between the baseof input transistor Q34b and the emitter of feedback transistor Q59.Capacitor 90 and capacitor 92 are selected to make the bandwidth ofreference input amplifier 86 roughly 1/10 that of input amplifier 52 bymoving the high frequency pole of reference input amplifier 36 toroughly 1/10 the frequency of the high frequency pole for inputamplifier 52. By cutting off the high frequencies of reference inputamplifier 86, the reference input amplifier does not act as a highfrequency noise source to the rest of receiver 16. A value of resistorR76 consistent with this example is 2240 ohms. In this example,resistors R43, R43b, R44, R44b, and R76 are all high resistivity (highrho) resistors since high rho resistors have less capacitance than lowrho resistors of equivalent resistance and take up less room on thechip.

Transistor Q34 is connected in a common emitter configuration and itsbase forms input 58 of input amplifier 52. Referring also to FIGS. 7, 8and 9, input amplifier 52 also includes a load resistor R32 driven bytransistor Q34, and a guard resistor R34 of different physicalcharacteristics from resistor R32 substantially surrounding loadresistor R32 on three sides wherein the distributed capacitance of loadresistor R32 is essentially all to resistor R34 and there is negligibledistributed capacitance between load resistor R32 and any otherelectrical component such as a ground plane. Referring in particular toFIG. 7, an integrated circuit according to the prior art is referred togenerally by reference numeral 93. The construction of integratedcircuit resistors in the prior art will typically begin with a P-typesubstrate 94 with an N+ buried layer 96 since monolithic integratedcircuit resistors are created by a process similar to the creation ofmonolithic integrated circuit transistors, and the N+ buried layer isused to provide a low resistance collector contact to the top side ofthe wafer in transistors. An N-type epitaxial layer 98 is then depositedover the N+ buried layer 96 and the P substrate 94. A P+ isolationdiffusion 100 is then added circumferentially to isolate the resistorfrom other elements, and an N+ sinker deposition 102 is deposited toconnect the N+ buried layer 96 to the surface of integrated circuit 93.Finally, a P-area 104 such as a double boron implant is added to the topof N-type epitaxial layer 98 located above the N+ buried layer 96. Inthis configuration, the N+ buried layer 96 is often referred to as adiffusion under film or DUF. In an integrated circuit such as integratedcircuit 93, P-type substrate 94 is grounded by a substrate ground 106.An electrical contact 108 is bonded to the end of P area 104 proximal tothe N+ sinker deposition 102, and a second electrical contact 110 isbonded to the end of P area 104 distal to N+ sinker deposition 102. Anelectrical contact 112 is also bonded to N+ sinker deposition 102 andelectrically connected to proximal electrical contact 108 by a wire 114.Thus, in the prior art, a resistor is formed between electrical contact108 and electrical contact 110 with P and N junctions back biased so asto not create any forward biased diode conditions, but with adistributed electrical capacitance along the length of P area 104between P area 104 and N+ buried layer 96. With N+ sinker deposition 102end of the N+ region electrically connected to the proximal end of Parea 104, that end of the N+ region is driven by the same voltage as thesame end of the resistor, but the opposite end of N+ buried layer 96 isessentially grounded so that the distributed capacitance between P area104 and N+ buried layer 96 gives a transmission line effect to theintegrated circuit resistor.

Referring now to FIGS. 8 and 9, an integrated circuit according to thepresent invention is referred to generally by reference numeral 115. Aresistor according to the present invention is constructed by depositingan N-type epitaxial layer 116 over a P-type substrate 118 with no buriedlayer in between. As in the prior art, a P+ isolation diffusion area 120encircles the resistor according to the present invention. Again, a Parea 122 is sunk into N-type epitaxial layer 116, but in the presentinvention an N+ sinker 124 is diffused into N-type epitaxial layer 116at one end of P area 122 and another N+ sinker deposition 126 isdiffused into N-type epitaxial layer 116 at the opposite end of P area122. An electrical contact 128 is bonded to N+ sinker deposition 124 andan electrical contact 130 is bonded to the respective end of P area 122.Similarly, an electrical contact 132 is bonded to N+ sinker deposition126 and an electrical contact 134 is bonded to its respective end of Parea 122. Back to FIG. 8, electrical contacts 132 and 134 are to bedriven by a common voltage or current, in this case the 3.9 voltreference voltage, and electrical contacts 128 and 130 are driven by acommon voltage or current, in this case by the collector of transistorQ34, either directly or through follower circuits such as followercircuit 66. In the present example, contact 130 is the electricalcontact to the desired P-type resistor R32 that would be degraded bydistributed capacitance. Contact 128 is driven with follower circuit 66to guard R32 and eliminate the effect of the distributed capacitance.

Referring again to FIG. 5, input amplifier 52 also includes at least onefollower circuit 66 for driving the voltage across guard resistor R34 bythe voltage across load resistor R32. Follower circuit 66 includescommon collector transistor Q38 and a level shifting diode D12. Theemitter voltage of transistor Q38 follows the collector voltage of inputtransistor Q34 but approximately one diode voltage drop lower due to thebase to emitter voltage drop of Q38. Level shifting diode D12 makes thevoltage applied across parallel resistor R34 track that applied acrossdistributed load resistor R32. In general, a method of altering theapparent electrical characteristics of a distributed electricalcomponent that inherently has a distributed shunt capacitance to someother electrical component such as a ground plane, such as a distributedelectrical resistance or even an inductance in a hybrid circuit,comprises placing a guard distributed electrical component of a similartype as the distributed electrical component substantially parallel tothe distributed electrical component between the component and theground plane and then driving the voltage across the guard distributedelectrical component by the voltage across the distributed electricalcomponent by means of a follower circuit. In the present case, thismethod makes load resistor R32 appear to the rest of the circuit as apure resistance without substantial distributed capacitance. Making theinput amplifier's load resistor appear to be a pure resistor gives avery fast response time for the input amplifier. A typical value forresistor R32 consistent with the example given is 7234 ohms, and forR32b is 4820 ohms in series with 1840 ohms of high rho resistor.

Making the quiescent emitter current of Q36 ten times as great as thequiescent emitter current for transistor Q34 greatly reduces the noisegenerated by input amplifier 52 by reducing the noise generated bytransistor Q36 since the noise voltage generated by a transistor is:##EQU4## where: e_(n) ² =the square of the noise voltage

T=°Kelvin

k=Boltzman's Constant

q=electronic charge

f_(BW) =noise bandwidth

I_(e) =emitter current of the transistor

The change in the voltage at the base of transistor Q34 with temperatureis determined by the dc feedback loop made up of transistor Q59,resistor R28, diode D10, resistor R30 and transistor Q30. Since it isdesirable to keep the dc voltage offset between the base of transistorQ34 and the base of transistor Q36 zero or at least very small, the 1.7volt regulated power supply is designed to have a temperaturecoefficient similar to that of the feedback loop, but since there is nofeedback mechanism to control the offset of transistor Q36, and sincethe emitter current of transistor Q36 is much greater than that oftransistor Q34, a special resistor-diode circuit is connected betweenthe emitters of the two transistors and ground. The resistor-diodecircuit consists of resistor R78 in parallel with the series combinationof resistor R80 and diode D48. The temperature coefficient of diode D48is opposite in sign from that of the two resistors, and therefore thechange in voltage drop across the diode helps offset the change involtage drop across the resistors. Typical resistance values consistentwith the present example are: R78=R78b=951 ohms, and R80=R80b=211 ohms.

Preamplifier 44 also includes a follower circuit 68 for following thevoltage across automatic gain control capacitor 62. Output 70 offollower 68 is thus isolated from the voltage across automatic gaincontrol capacitor 60 and does not drain the charge from the capacitor.The difference in voltage between output 56 and output 70 constitutes abipolar electrical output, that is electrically bipolar in that itsvoltage or current representation swings in near equal amplitude about areference value, so that preamplifier 44 generates bipolar electricaldata corresponding to the unipolar optical data received.

It can now be seen that light is received by light sensitive device 50,creating a unipolar signal which is amplified by input amplifier 52.Automatic gain control circuit 54 acts on input amplifier 52 to adjustthe gain according to the input signal strength. The signal at theoutput of input amplifier 52 is averaged by the RC circuit made up ofresistor R26 and AGC capacitor 62. The signal at output 56 of inputamplifier 52 and the averaged signal of output 56 as it appears acrossAGC capacitor 62 and at the output of follower circuit 68 create bipolarinputs to post amplifier 46.

Under some circumstances LED 26 emits too much light or light sensitivedevice 50 has too high an amount of "dark" leakage current. Hightemperature operation is one example of the latter. Under suchcircumstances, preamplifier 44 gain is lowered too much for properoperation. One embodiment of preamplifier 44 particularly well suitedfor operating under circumstances of high dark leakage or too much lightbreaks the connection between the base of transistor Q32 and AGCcapacitor 62. Leads are brought off the chip from both the base oftransistor Q32 and AGC capacitor 62. The two leads can be connectedtogether during normal operation. When operating under conditions ofconstant high current from light sensitive device 50, the lead from thebase of transistor Q32 is connected to the same voltage as the base oftransistor Q30, in this case the 1.7 volt regulated voltage supply. Inthis way, the effective feedback resistance is chosen in advance.

Referring to the loop in preamplifier 44 made up of input amplifier 52,resistor R28, diode D10, resistor R30, and transistor Q30, preamplifier44 is critically damped with two poles to provide loop stability withvariations in input. One major pole is at the input node, being that ofload resistor R32 and the base-to-emitter capacitance of transistor Q34.The time constant of the base-to-emitter capacitance of transistor Q34tied to load resistor R32 nominally equals that of the feedback loop sothat the pole cancels the zero created by the feedback loop.

The loop is also stable with temperature. Loop stability is independentof the dc open loop gain. It is desirable to have the frequency at whichloop gain equals one to not vary with temperature. Since r_(e)=kT(qI_(e)), where r_(e) is the emitter resistance for the inputtransistor which is constant, dc emitter current I_(e) is proportionalto absolute temperature.

Referring now to FIG. 6, post amplifier 46 amplifies the bipolarelectrical data received from preamplifier 44 while also limiting itsown output voltage swing in order to prevent saturating its amplifierstages and distorting the electrical signal. Postamplifier 46 includesat least one amplifier 72 for receiving digital data of varyingamplitude from preamplifier 44 and transmitting bipolar digital data ofrelatively constant amplitude corresponding to the data received.Amplifier 72 includes at least one balanced differential transistorstage including a first transistor Q40, a second transistor Q42, loadresistors R48 and R50, current source 71 and a means between the outputof the first transistor and the output of the second transistor forlimiting the difference in voltage between the two outputs to apredetermined amount, the predetermined amount being small compared tothe voltage of the at least one power supply, in this case compared tothe roughly five volt power supply and the 3.9 volt regulated powersupply. The means for limiting the difference in voltage between the twooutputs comprises a first diode D14 connected in a forward biaseddirection from the output of transistor Q40 to the output of transistorQ42, and a second diode D16 connected in a forward biased direction fromthe output of transistor Q42 to the output of transistor Q40.

In a preferred embodiment, postamplifier 46 comprises two differentialamplifier stages 72 and 74, in tandem wherein one output 73 ofdifferential transistor stage 72 is resistively coupled to both inputsof differential transistor stage 74 through equal resistors R36 and R38,and the other output 75 of differential transistor stage 72 iscapacitively coupled to only one input of differential transistor stage74 through capacitor 76. Output 73 is taken from the emitter followermade up of transistor Q45 and resistor R52, and output 75 is taken fromthe emitter follower made up of transistor Q43 and resistor R54, so theoutputs of differential transistor stage 72 correspond to the voltagestaken at the collectors of transistors Q42 and Q40 respectively. By dccoupling output 73 through resistors R36 and R38 into both inputs ofdifferential transistor stage 74, the inputs of stage 74 are biased tothe same level. The ac coupling of output 75 through capacitor 76 feedsone half of stage 74 and effectively swings the total signal. Theconventional way to ac couple two differential stages such as stages 72and 74 is to capacitively couple output 73 to the base of Q40b andcapacitively couple output 75 to the base of Q42b while connecting equalresistances from the base of Q40b to ground and from the base of Q42b toground. The conventional method of ac coupling thus uses an additionalcapacitor which normally must be connected externally to the chip.Differential transistor stage 74 can be of construction similar to thatdescribed for differential transistor stage 72 with transistor Q40b,transistor Q42b, load resistor R48b, load resistor R50b and currentsource 71b corresponding to transistor Q40, transistor Q42, loadresistor R48, load resistor R50, and current source 71 respectively.

Output stage 48 accepts amplified balanced electrical data frompostamplifier 46. It then shapes the data and generates single-endedelectrical data which is compatible with TTL logic. In one embodiment,output stage 48 includes a current mode comparator 78 for translatingamplified bipolar electrical data from postamplifier 46 to a signalreferenced to ground, and logic circuitry 80 for shaping the electricaldata, driven by current mode comparator 78 whereby the conductedelectrical data at output 82 of logic circuitry 80 corresponds to theconducted electrical data received by transmitter 14.

Differential amplifier 74 includes two common collector outputtransistors Q44 and Q46 which drive comparator 78 directly by theiremitter voltages. Comparator 78 includes two current mode switches, onemade up of transistors Q48 and Q50 and diode connected transistor Q52,and the other current mode switch is made up of transistors Q54 and Q56and diode connected transistor Q58. Diode connected transistor Q52connects the base of transistors Q48 and Q50 to ground so thattransistors Q48 and Q50 match their emitter currents to that of Q52 byforming a current mirror. The other current mode switch is similarlyarranged. Comparator 78 also includes a resistor R45 connected betweenthe emitter of transistor Q44 and diode connected transistor Q52 and aresistor R46 connected between the emitter of transistor Q46 and diodeconnected transistor Q58. The voltage output of output transistor Q46minus one diode drop is dropped across resistor R46 to provide a currentfor driving the bases of transistors Q54 and Q56 in one current modeswitch and the collector of transistor Q48 in the other. Similarly, thevoltage output of output transistor Q44 minus one diode drop is droppedacross resistor R45 to provide current for driving the bases oftransistors Q48 and Q50 in one current mode switch and the collector oftransistor Q54 in the other. Thus, when the output of Q46 is high, ittends to switch Q54 and Q56 low while Q44 does not provide sufficientbase current to switch Q48 and Q50 low. The switch is reversed when theoutput of Q44 is high and the output of Q46 is low. Schottky diodes D18,D20, D22, and D24 are connected from the collectors of Q48, Q50, Q54,and Q56 respectively to the same voltage level as the bases of thetransistors, a single diode voltage drop above ground by diode D28 toprevent the current mode switch transistors from switching too hard intosaturation and thus shortening or stretching pulses. Current modecomparator 48 also includes equal resistors R40 and R42 connectedbetween the at least one power supply and the collector of Q50 and Q56respectively for converting the current outputs of current modecomparator 78 into voltage outputs for driving the output logic gate.Diode D26 is included between resistor R42 and the collector oftransistor Q56 in order to provide an additional diode drop forinterfacing with logic circuitry 80.

A preferred arrangement of logic circuitry 80 for converting thedifferential output of current mode comparator 78 into a form compatiblewith TTL logic includes output transistors Q60 and Q62 connected in a"totem pole" arrangement with resistor R54 connected between powersupply 12 and the collector of transistor Q62. Output 82 is taken at thejunction of the emitter of transistor Q62 and the collector oftransistor Q60. Transistor Q62 is the second stage of a darlington pairformed with transistor Q64 and resistor R56. Logic circuitry 80 alsoincludes pull-up transistor Q66 driven by the differential output ofcurrent mode comparator 78 at the collector of transistor Q50, andpull-down transistor Q68 driven by the other differential output ofcurrent mode comparator 78 at the collector of transistor Q56. Thecollector of transistor Q68 and the emitter of transistor Q66 are bothconnected to the base of output transistor Q60. The emitter oftransistor Q68 is connected to ground, whereas the collector oftransistor Q66 is connected to the base of transistor Q64 and toresistor R58 which is connected to power supply 12. Transistors Q60,Q64, Q66, and Q68 are all clamped with Schottky diodes D30, D32, D34,and D36 respectively. When the current mode comparator 78 outputconnected to the base of transistor Q66 is high, the current modecomparator output at the base of transistor Q68 is low so thattransistor Q66 is conducting and transistor Q68 is not, thus pulling upon the base of transistor Q60 and pulling down on the base of transistorQ64 and hence on the base of transistor Q66 and the emitter oftransistor Q66. Pulling up on the base of transistor Q60 and down on thebase of transistor Q62 brings the voltage on output 82 low. When thedifferential outputs of current mode comparator 78 are reversed, on theother hand, making the base of transistor Q68 high and the base oftransistor Q66 low, transistor Q66 turns off, its collector going high,and transistor Q68 tries to switch on, but since it cannot get currentthrough transistor Q66, it takes the charge back out of the base oftransistor Q60, thus ensuring quick turn-off of transistor Q60. Thecollector of transistor Q66 going high makes the base of transistor Q64and the base of transistor Q62 both go high, pulling up the voltage atoutput 82. Typical resistance values consistent with the present exampleare: R54=110 ohms, R56=3.5K ohms high rho, and R58=2K high rho. Forlarge values of resistances, it is often advantageous to accept thelarger temperature coefficient of the high rho resistor in order toreduce the amount of space taken up on the chip.

Referring again to FIG. 4, one embodiment of receiver 16 also includes astatus detector 138 for detecting the presence of a signal in the formof conducted electrical data. In the absence of a signal, receiver 16will generate an output signal at output 82 which switches on noisewithin the receiver. Status detector 138 determines if conductedelectrical data signal is actually present, and in the example shown,generates a positive signal at status detector output 140 if such asignal is present. Output 140 is then logically anded with receiveroutput 82 by and gate 142. As shown, and gate 142 is off the chip, butit could also be on the chip. The signal at output 144 of and gate 142thus corresponds to the optical data received by light sensitive device50, the signal remaining low when no optical data is received by lightemitting device 50. Status detector 138 includes a means 146 for sensingthe presence of conducted electrical data at the output of amplifierstage 74, status capacitor 148 external to the chip for smoothing outspikes in the output of means 146 in order to present a dc voltagelevel, level shifter 150 for shifting the voltage levels to a rangecompatible with the logic used such as TTL, and logic circuitry 152 fordriving output 140.

Referring to FIG. 10, means 146 for sensing the presence of conductedelectrical data includes a differential amplifier made up of transistorsQ70 and Q72 driven by the emitters of transistors Q44 and Q46 ofamplifier 74, emitter resistors R60 and R62 associated with transistorsQ70 and Q72 respectively, and load resistors R64 and R66, as well ascurrent source 154 connected between the two emitter resistors R60 andR62. In the present example, current source 154 draws approximately twomilliamps. Means 146 also includes clamping transistors Q74 and Q76cross-coupled in the collectors of transistors Q70 and Q72 of thedifferential stage, and load resistor R68 connected between power supply12 and the collectors of Q74 and Q76. In the present example, theamplitude of the differential signal between Q44 and Q46 is 0.4 voltsfor a peak-to-peak input current of 125 nanoamps. The gain of thedifferential amplifier portion of means 146 is 1.9 and substantiallyconstant over temperature and power supply variations. This gainproduces a signal of 0.76 volts which is equal to the base-to-emittervoltage required to turn on either of clamping transistors Q74 or Q76.Since the collectors of cross-coupled transistors Q74 and Q76 areconnected together such that when either is turned on, current throughtheir load resistor R68 drops a voltage across R68, a full-waverectified signal is produced. The node between the collectors of Q74 andQ76 and load resistor R68 is connected to status capacitor 148. Typicalvalues for the resistors of means 146 consistent with the presentexample are: R60=R62=174 ohms; R64=R66=760 ohms; and R68=1.5K ohms.

Level shifter 150 includes an input emitter follower made up oftransistor Q78 and resistor R70. Level shifter 150 also includes avoltage-to-current translator comprising transistor Q80, current sourceoutput transistor Q82 and resistor R72 connected between the emitter oftransistor Q80 and the collector of current source output transistorQ82. The collector of transistor Q80 is connected to power supply 12,and the emitter of transistor Q82 is connected to ground. The biascurrent for the base of transistor Q82 is supplied by the seriesconnection of resistor R74, and diodes D38, D40, and D42 connectedbetween power supply 12 and ground. The base of transistor Q82 isconnected between diodes D40 and D42, making the base of transistor Q82one diode voltage drop above ground. Current source output transistorQ82 is Schottky clamped by Schottky diode D44 to prevent saturation. Theoutput of level shifter 150 taken at the node between resistor R72 andthe collector of Q82 becomes the input to logic circuitry 152.

Logic circuitry 152 comprises a single inverter which includestransistor Q84 which is Schottky clamped by diode D46, and load resistorR76 connected between the collector of transistor Q84 and power supply12. Since the emitter of transistor Q84 is connected to ground and itsbase is connected to the collector of transistor Q82, thecollector-to-emitter voltage of transistor Q82 remains onebase-to-emitter voltage drop. The current through resistor R72 thusbecomes equal to (the voltage across status capacitor 148 minus thebase-to-emitter voltages of Q78, Q80, and Q84) divided by the resistanceof R72. The current through diode D42 which is mirrored in the emitterof transistor Q82 equals (the voltage of voltage source 12 minus 3 diodevoltage drops) divided by the resistance of R74. The current throughresistor R72 minus the current through transistor Q82 is the currentthrough the base of transistor Q84, and the amount of that currentdetermines whether the collector of transistor Q84 will be high or low.For the example given and a voltage source 12 of 4.5 volts, the base oftransistor Q78 sees a dc level of about 3 volts when cross-coupledtransistors Q74 and Q76 are turned on. If the cross-coupled transistorsare turned off due to too low an input current, then the base oftransistor Q78 is at the power supply potential of 4.5 volts. Thethreshold of the TTL output is set such that when status capacitor 148has discharged one time constant, it will switch to a "one" output. Withan R68 of 1.5K ohms, this means 1.5 nanoseconds per picofarad ofexternal capacitance. For a ten megabit per second data rate, the bittime is 100 nanoseconds. Arbitrarily picking three bit times for theresponse, results in a 200 picofarad external capacitor. For a 10K bitper second data rate and three bit times, this means a 0.2 microfaradcapacitor. Typical resistor values consistent with the present examplefor R70, R74, R72, and R76 are 3K ohms, 2.2K ohm, 1.2K ohm and 4K ohmrespectively.

Without the use of status detector 138, receiver 14 is limited tosystems that produce a signal of sufficient amplitude continuously. Byusing status detector 138, receiver 16 can be run asynchronous up to1/10 of the clock rate without refreshing the most recently receivedsignal. By use of status detector 138 it is possible to have an up toten megabit circuit that can operate at any lower data rate with a tenmegabit sensitivity. The second post amplifier output is used as theinput to means 146 for sensing the presence of conducted electrical databecause the second post amplifier, post amplifier 74, has the highestlevel linear signal and makes the sensing relatively straightforward.Because the sensing is a linear function performed at a relatively highvoltage level, it must be level shifted to TTL levels in preparation forthe open collector TTL output. The TTL output includes an on-chippassive load.

Because of the presence of noise in receiver 16 which is proportional tothe square of the frequency of the noise, receiver 16 must have at leasttwo poles in the high-frequency cutoff characteristic so that the totalnoise will be bounded. The low frequency pole is at f_(o) and is:

    f.sub.o =1(2R.sub.f C.sub.f)=13.5 MHz (in the present example)

where:

R_(f) =effective feedback resistance (varies by a factor of 20 over thedynamic range in the present example)

C_(f) =effective feedback capacitance

The second pole is in post amplifier 46 and is:

    f.sub.2 =1.85f.sub.o =25 MHc (in the present example)

In summary, conducted electrical data is received by receiver 14 eitherthrough input gate 36 if the data is already Manchester encoded orthrough Manchester encoder 37 if the data is in NRZ form. Manchesterencoded conducted electrical data then drives output driver 28 which inturn drives light emitting device 26. LED 26 emits radiated wave datacorresponding to the conducted electrical data of output driver 28. Theradiated wave data is transmitted through fiber optic data transmissioncable 18 and is received by receiver 16 by means of light sensitivedevice 50. Light is inherently unipolar, but when the data is receivedby light sensitive device 50, it is converted back into conductedelectrical data which is converted into a bipolar form by preamplifier44. By converting to a bipolar signal referenced about a point somewherein the mid-range between the power supply voltages, in this case powersupply voltages of 3.9 volts and ground, there is less signaldegradation of small amplitude signals. The small signal is then furtheramplified by a post amplifier 46 which brings the signal up to anamplitude which can be compared by comparators 78. The compared signalof comparator 78 is then converted by logic stage 80 to a unipolarsignal which is compatible with the logic to be used. In a preferredembodiment, the final output of logic stage 80 is anded with the outputof a status detector 138 to ensure that the final signal representsactual data and that receiver 16 is not switching on noise.

The reference voltages used by the receiver are provided by circuitrysimilar to temperature compensated voltage reference circuit 24 fortransmitter 14, but are designed for zero temperature coefficientwhereas temperature compensated voltage reference 24 is designed for atemperature coefficient to match that of plurality of resistors 32.

The specific embodiment illustrated of receiver 16 is adapted for usewith data that has an approximately fifty percent duty cycle such as aManchester code, but the principles of the invention can be appliedgenerally. Although a specific embodiment has been described for fiberoptic transmission using an LED and a photodiode, both the transmitterand receiver will work for atmospheric transmission or wave guidetransmission. Both circuits also lend themselves well to microwavetransmission.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims. As many possible embodiments may be made of the inventionwithout departing from the scope thereof, it is to be understood thatall matter herein set forth or shown in the accompanying drawings is tobe interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed is:
 1. An amplifierhaving an output, comprising in combination:a high gain, low noisedifferential input stage including inverting input transistor means andnon-inverting second transistor means, the base of the second transistormeans to be controlled by a predetermined voltage wherein the emittercross-sectional area of the second transistor is significantly greaterthan the emitter cross-sectional area of the input transistor; means forbiasing the input transistor means and the second transistor means tosubstantially equal base to emitter voltages, thereby dividing thequiescent emitter current between the input transistor means and thesecond transistor means in substantially the same ratio as the emittercross-sectional areas; and means for providing feedback from the outputto the base of the input transistor means.
 2. An amplifier according toclaim 1 wherein the emitter cross-sectional area of the secondtransistor is approximately ten times greater than the emittercross-sectional area of the input transistor.
 3. An amplifier accordingto claim 1 further comprising a signal source connected to the base ofthe input transistor means.
 4. An amplifier according to claim 3 whereinthe signal source is a photo detector.
 5. An amplifier according toclaim 1 to be used with means for generating a reference voltage,wherein the means for providing feedback comprises in combination:firstimpedance means connecting the output to the base of the inputtransistor; and second impedance means connecting the reference voltageto the common connection of the first impedance means and the inputtransistor means.
 6. An amplifier according to claim 1 to be used withmeans for generating a reference voltage, wherein the means forproviding feedback comprises in combination:a capacitor, wherein thevoltage across the capacitor is responsive to the voltage output of theamplifier averaged over a period of time; a first current divider branchconnected to the base of the input transistor means and having apredetermined quiescent current; resistance means connecting thereference voltage to the common connection of the first current branchand the input transistor means for providing current to the firstcurrent divider branch; and a second current divider branch connected tothe first current divider branch at the end opposite the resistancemeans and the input transistor means, wherein the current in the secondcurrent divider branch is controlled by the voltage across thecapacitor, the quiescent current in the second branch beingsignificantly greater than the quiescent current in the first branch,whereby the combination of the capacitor, the first current dividerbranch and the second current divider branch act as an automatic gaincontrol circuit connected in a feedback relationship to the amplifier.7. An amplifier according to claim 6 further comprising a signal sourceconnected to the base of the input transistor means.
 8. An amplifieraccording to claim 7 wherein the signal source is a photo detector. 9.An amplifier having an output, comprising in combination:a high gain,low noise differential input stage including inverting input transistormeans and non-inverting second transistor means to be controlled by apredetermined voltage; means for biasing the input transistor means andthe second transistor means wherein the quiescent current issubstantially different in the second transistor means from thequiescent current in the input transistor means; and means for providingfeedback from the output to the base of the input transistor means. 10.An amplifier according to claim 9 further comprising a signal sourceconnected to the base of the input transistor means.
 11. An amplifieraccording to claim 10 wherein the signal source is a photo detector. 12.An amplifier according to claim 9 to be used with means for generating areference voltage, wherein the means for providing feedback comprises incombination:first impedance means connecting the output to the base ofthe input transistor; and second impedance means connecting thereference voltage to the common connection of the first impedance meansand the input transistor means.
 13. An amplifier having an output, to beused with means for generating a reference voltage, comprising incombination:a high gain, low noise differential input stage includinginverting input transistor means and non-inverting second transistormeans to be controlled by a predetermined voltage; means for biasing theinput transistor means and the second transistor means wherein thequiescent current is substantially different in the second transistormeans from the quiescent current in the input transistor means; acapacitor, wherein the voltage across the capacitor is responsive to thevoltage output of the amplifier averaged over a period of time; a firstcurrent divider branch connected to the base of the input transistormeans and having a predetermined quiescent current; resistance meansconnecting the reference voltage to the common connection of the firstcurrent branch and the input transistor means for providing current tothe first current divider branch; and a second current divider branchconnected to the first current divider branch at the end opposite theresistance means and the input transistor means, wherein the current inthe second current divider branch is controlled by the voltage acrossthe capacitor, the quiescent current in the second branch beingsignificantly greater than the quiescent current in first branch,whereby the combination of the capacitor, the first current dividerbranch and the second current divider branch act as an automatic gaincontrol circuit connected in a feedback relationship to the amplifier.14. An amplifier according to claim 13 further comprising a signalsource connected to the base of the input transistor means.
 15. Anamplifier according to claim 14 wherein the signal source is a photodetector.